I think the word abandoned here is a bit premature. People have to realize because of changes in the core of OMV a lot of the plugins were ported 2 times for OMV 3. Not fun. So I think Aaron and HK-47, who have done most of the porting, are feeling a bit burned out. After most of the plugins are ported they will probably get bored and do a few more. But some of the plugins are just not being used enough to justify them being maintained. And those will likely not be ported.

I don't see any work being done on the docker plugin as of yet. But just because it is not on Github does not mean that the person working on it has not started the porting for OMV 3.

it is a nice informative therd... many thanks for all the people behind omv and the omv-extras plugins
for a information i post my used extras-plugins:
mysql
nginx
docker
emby
fail2ban
flashmemory
autoshutdown
rsnapshot
letsencrypt

Though the plugin system is one of the strong points of OMV compared to other NASes, I appreciate you don't want to waste time that could have been spent better.
I guess installing Nextcloud on the new base with a newer PHP version should be straightforward.